Display substrate electrodes with auxiliary metal layers for enhanced conductivity

ABSTRACT

Disclosed is a transparent substrate element for electronic displays that includes a transparent base layer, a plurality of independently addressable transparent electrodes disposed on the base layer, and a contiguous metallic coating associated with each transparent conductive electrode to increase the conductivity of the associated transparent conductive electrode, wherein the contiguous metallic coating comprises a periodic array of holes arranged to allow a significant amount of visible light to be transmitted through the substrate element.

This is a divisional of application Ser. No. 09/076,165 filed May 12,1998 now U.S. Pat. No. 6,037,005.

FIELD OF THE INVENTION

This invention relates to display substrates having transparentconductive electrodes with auxiliary metal layers to increase theirconductivity and to a method of providing these auxiliary metal layerswithout high precision alignment steps.

BACKGROUND OF THE INVENTION

Transparent conductive oxide (TCO) films are used in many display deviceapplications where electric fields must be applied to activate pictureelements (pixels) and where optical transparency is essential. Forexample, liquid crystal display substrates often employ parallel stripsof TCO material as electrodes. When a pair of such substrates arecombined in a display with their opposing TCO electrode strips orientedto form a matrix, the area of the display through which any pair of TCOstrips cross defines a pixel. By applying an electric field between apair of crossed TCO strips, the liquid crystal disposed therebetween maybe reoriented. This reorientation affects how light is transmittedthrough these activated areas. For example, polarized light travelingthrough a liquid crystal display will be transmitted through activatedpixel areas with a polarization perpendicular to that transmittedoutside of the activated pixel areas. Polarizers may then be employed sothat the display appears dark in the activated pixel areas while lightis transmitted through the display elsewhere.

The speed at which pixels can be activated and deactivated dependscritically on the conductivity of the TCO electrodes. A shorter “refreshrate,” corresponding to the speed at which pixels can be turned on andoff, may be required in many applications including those employinglarge or high resolution displays. Shorter refresh rates may be realizedby increasing the conductivity of the TCO electrodes, especially indisplays having a high pixel density. Increasing the conductivity of TCOelectrodes also enhances the display appearance by improving uniformity.

One way in which to increase the conductivity of a TCO layer is toanneal it at high temperatures (above about 250° C.). When glass is usedas the substrate material, this method is viable. However, in manyapplications such as large area liquid crystal displays, glasssubstrates are too heavy, and so polymeric substrates are preferred.Polymer materials suitable as substrate materials in liquid crystaldisplays often have glass transition temperatures and meltingtemperatures well below the high temperatures required for annealing toincrease the conductivity of TCO layers. As such, high temperatureannealing is not an available option when attempting to increase theconductivity of the TCO electrodes when polymeric substrates areemployed.

Another means of increasing the conductivity of a TCO layer is toprovide an auxiliary metal layer in contact with the TCO layer.Typically, the metal layer takes the form of a narrow strip, or line, ofmetallic material deposited on a TCO electrode. Addition of a metalstrip increases the conductivity of a TCO electrode by decreasing theresistivity according to the following relationship:${R_{T} \propto \frac{R_{TCO} \times R_{M}}{R_{TCO} + R_{M}}},$

where R_(T) is the resistivity of the electrode as a whole, R_(TCO) isthe resistivity of the TCO layer, and R_(M) is the resistivity of themetal strip. When R_(M) is much less than R_(TCO), which is typicallythe case, R_(T) approaches R_(M), thus resulting in an electrode havinga resistivity much lower than that of a bare TCO electrode. The increasein conductivity results as long as the metal layer is continuous alongthe length of the electrode. This is significant because high densitydisplays having small pixels and thereby small electrodes require smallauxiliary metal layers that may be amenable to cracks or fractures thatdisrupt electrical conductivity.

Because transparency of the final device is often essential, and becausemetal layers thick enough to enhance the TCO electrode conductivity aregenerally optically opaque, it is important that the metal strip doesnot substantially cover the TCO electrode. Moreover, when independentlyaddressable TCO electrode strips are arranged in close proximity on asubstrate, alignment of each metal strip with each TCO strip isessential. Without alignment, the metal strips may cross over toadjacent TCO electrodes, thereby causing an electrical short acrossadjacent electrodes. Alignment is especially critical on large area andhigh resolution displays, where the electrode strips may be longer orcloser together, thus leaving less room for error.

Typically, one of three methods (or a variation thereof) are used tofabricate TCO electrodes having auxiliary metal strips. First, thinmetal strips may be directly deposited through a mask onto preexistingTCO electrode strips. This requires precision alignment of thedeposition mask with the patterned electrodes. Second, metal strips maybe deposited on substrate having a TCO layer that has not yet beenpatterned into electrodes. Portions of the TCO layer and any unwantedmetal are then removed to form TCO electrodes with auxiliary metalstrips. This requires precision alignment of an etch mask or a laserscribe with the patterned metal strips. Lastly, metal strips may bedeposited directly onto a substrate followed by deposition of TCO stripsdirectly on top of the metal strips. Again, this requires precisionalignment of a deposition mask with the patterned metal strips. In eachof these methods, the required precision alignment step reduces theefficiency of the process and risks introduction of defects.

A method for providing auxiliary metal strips to TCO electrodes withouta precision alignment step is described in U.S. Pat. No. 5,342,477 toCathey. This method involves providing a substrate having a plurality oftransparent electrodes, each electrode comprising a strip of transparentsilicon dioxide stacked on a strip of transparent conductive material.The entire surface is then coated with a highly conductive material. Thehighly conductive material is then vertically etched until the materialon top of the electrodes is removed and an area between the electrodesis exposed. What remains is a “runner” of conductive material along eachside of the transparent electrode stack.

While the method disclosed by Cathey does not require a high precisionalignment step, it has major deficiencies affecting its viability.First, the method relies on relatively thick electrode stacks so thatconductive material will accumulate at the edges of the electrodesduring deposition and remain there after the etching step. While thestack is substantially transparent, it is well known that increasing theelectrode thickness will decrease the brightness of the display. Second,the conductive “runners” contact only the sides of the transparentconductive portions of the electrode. Because the transparent conductiveportions must be thin, the total area of surface contact between theconductive runners and the transparent conductive strips is quite small.Thus, delamination of the conductive runners from the transparentconductive strips is likely. When delamination occurs, the conductiverunners have no effect.

Another method for providing auxiliary metal strips to the edges of TCOelectrodes without a precision alignment step is discussed in JapaneseKokai Patent Application No. 4-360124. In the method there disclosed,TCO electrodes are formed on a substrate by conventionalphotolithography. The photoresist is left on the TCO material, and ametal is electroplated onto the exposed side edges of the electrodes.The photoresist is then removed to leave a series of TCO electrodeshaving metal strips along their edges. While this method addresses someof the deficiencies of the Cathey method, the reliance on metal platingtechniques risks excessive metal build-up between electrodes that wouldshort-out adjacent electrodes in the display. This risk is especiallyapparent for high density displays where the distance between electrodesmay be quite small.

SUMMARY OF THE INVENTION

The present invention addresses these shortcomings by providing a methodof patterning auxiliary metal layers to increase the conductivity ofindependently addressable TCO electrodes without the need for precisionalignment in relation to pre-patterned electrodes.

In one embodiment, the method of the present invention first involvesproviding a substantially transparent substrate having a transparentconductive layer thereon. Next, parallel strips of resist material areformed on the transparent conductive layer, thereby leaving areas of thetransparent conductive layer uncovered by the resist material. Acollimated metal beam is then used to deposit a metal coating on thetransparent conductive layer and strips of resist, whereby thecollimated beam is incident at such an angle as to be partially blockedby the parallel strips of resist material. This leaves portions of theuncovered transparent conductive layer exposed. The exposed portions ofthe transparent conductive layer are then removed. Finally, the stripsof resist material are removed, along with any metal coating thereon, toform a plurality of independently addressable electrodes on thesubstrate, each having an auxiliary metal strip.

In another embodiment, the method of the present invention firstinvolves providing a substantially transparent substrate. Next, parallelstrips of resist material are formed on the transparent substrate. Ametal is then deposited on the substrate and resist strips, making surethat metal material is deposited at the sides of the resist strips. Themetal coating is then etched to remove it from the tops of the resiststrips and from the area of the substrate between the resist strips, butnot from the sides of the resist strips. The metal coating that remainsforms continuous metal lines along the edges of the resist strips. Next,a transparent conductive film is deposited to substantially cover theresist strips, metal lines, and exposed substrate. Finally, the resiststrips are removed. This leaves parallel strips of the transparentconductive film on the substrate, each of the parallel strips oftransparent conductive film having its sides bordered by the continuousmetal lines. Each parallel strip of transparent conductive film borderedby the metal lines represents an independently addressable electrode onthe surface of the substrate, each of the electrodes separated by adistance that corresponds to the width of the initially formed resiststrips.

In still another embodiment, the method of the present inventioninvolves providing a substantially transparent substrate having atransparent conductive layer thereon. A contiguous metallic coating isformed adjacent to the transparent conductive layer, either on thesurface opposing the substrate or between the transparent conductivelayer and the substrate. The contiguous metallic coating ischaracterized by a periodic array of holes, such as a hexagonal array ofcircular holes or a regular array of diamond-shaped holes. The holesensure that the substrate element remains substantially transparent.Independently addressable transparent electrodes are then formed byremoving portions of the transparent conductive layer and any metalliccoating adjacent to those portions.

The present invention also includes a substrate element suitable for usein an electronic display device. The substrate element includes asubstantially transparent substrate and a plurality of independentlyaddressable electrodes. The electrodes are made up of a layer of atransparent conductive material and a contiguous metallic coating havinga periodic array of holes.

It is the purpose of the present invention to provide a method ofincreasing the conductivity of TCO electrodes by providing auxiliarymetal layers adjacent to the TCO electrodes without the need forprecision alignment of the metal layers prior to their formation. It isfurther the purpose of the present invention to provide a method ofincreasing the conductivity of TCO electrodes on liquid crystal displaysubstrates by providing auxiliary metal layers adjacent to the TCOelectrodes. It is further the purpose of the present invention toprovide a display substrate element having independently addressabletransparent electrodes comprising a transparent conductive layer and apatterned metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(e) are schematic representations of the steps involved in aparticular embodiment of the method of the present invention.

FIGS. 2(a)-(g) are schematic representations of the steps involved inanother embodiment of the method of the present invention.

FIGS. 3(a)-(f) are schematic top and side views representing the stepsinvolved in a particular embodiment of the method of the presentinvention.

FIGS. 4(a)-(e) are schematic representations of various metal coatingpatterns that may be employed in the present invention.

DETAILED DESCRIPTION

The present invention involves a method for providing transparentconductive electrodes with auxiliary metal layers to enhance theconductivity of the electrodes without significantly altering theirtransparent properties. The metal layers are provided without the use ofa high precision alignment step such as alignment of a deposition oretch mask with a pre-patterned electrode structure. While the metallayers provided in the present invention may be any conductive metallicmaterial suitable for the particular application, the metal layers arepreferably include materials capable of being deposited onto a substratevia known techniques such as evaporation or sputtering. Examples of suchmaterials include, but are not limited to, Cr, Cu, Ag, Au, Ni, W, Al,Pt, Ti, Fe, Sn, or combinations or alloys thereof.

While the various aspects of the present invention are best understoodin light specific embodiments, the embodiments described hereinafter andthe examples contained therein are in no way meant to limit the scope ofthe present invention or its recited claims.

A. First Embodiment

One embodiment of the method of the present invention is shown in FIGS.1(a)-(e). First, a substrate element is provided having a substrate 20with a transparent conductive layer 22. The substrate 20 is asubstantially transparent material suitable for use in electronicdisplay devices. Preferred substrates include glass and any suitabletransparent polymeric material. The transparent conductive layer 22 ispreferably a transparent conductive oxide (TCO) material, and the TCOmaterial is preferably indium tin oxide (ITO).

Next, strips 24 of resist material are provided as shown in FIG. 1(b).The width w of the resist strips 24 corresponds to the width of a pixelin a finished display. The resist strips 24 are provided by coating thesurface with a resist material and drying it to form a coating having athickness h. The resist coating may then be exposed to light of acertain wavelength to activate the photo-initiators in the resist. Theresist coating is exposed to the light through a mask so that onlycertain areas are activated. Depending on whether the resist is apositive resist or a negative resist, the activated or non-activatedareas may be removed by rinsing in a solution. The resulting resistcoating is a plurality of resist strips separated by a distance P thatcorresponds to the inactive area between pixels in the completed displaysubstrate. The resist material may be any resist material desired thatdoes not adversely react with the electrode material. There are manycommercially available formulations of resist materials, and theprocedure for their coating, drying, activation, and removal will beknown to the user when a particular resist material is chosen.

A metal coating 26 is then deposited over the substrate as shown in FIG.1(c). In order to form narrow metal strips at the TCO surface, atechnique called “shadow coating” is employed. In shadow coating, acollimated beam 28 of the material to be deposited is formed, usually ina vacuum chamber using known deposition techniques. The collimated beam28 is then directed at a certain angle of incidence 30 relative to theplane of the substrate element. The resist strips 24 function to block aportion of the collimated beam so that only a strip of width d is formedin the area between the resist strips. The width d of the metal coatingon the TCO surface is given by:${{\tan \quad \theta} = \frac{h}{P - d}},$

where θ is the angle 30 of the collimated beam measured in reference tothe plane of the substrate element. The step of shadow coating leaves anarea of the TCO layer exposed having a width of P-d as shown in FIG.1(c).

The exposed portions of the TCO layer 22 are then etched, using themetal coating as an etch barrier. The result is shown in FIG. 1(d);

The resist strips 24 are then removed by lift-off, along with any metalcoating 26 residing on the resist. The resulting substrate element,shown in FIG. 1(e), includes a substrate 20 having a plurality ofindependently addressable electrodes, each electrode having a TCO layer22 and an auxiliary metal layer 26. The auxiliary metal layers increasethe conductivity of the TCO electrodes. In addition, the auxiliary metallayers are often thick enough to be opaque and thus function as a darkmatrix.

Dark matrix is opaque material on a display substrate that blocks lightfrom being transmitted through the inactive areas of the display. Thisincreases the contrast of the display. When two or more adjacent pixelsare activated, thus appearing dark, the absence of a dark matrix allowslight to be transmitted through the inactive areas between the activatedpixels. This makes the activated pixels appear gray rather than black,thus reducing contrast. The presence of a dark matrix alleviates thisconcern.

Dark matrix may be applied during the process shown in FIG. 1. After thestep of removing the exposed TCO layer (shown in FIG. 1(d)), anon-conductive opaque material may be deposited over the surface. Thus,when the resist strips are removed, the resulting substrate element willbe as shown in FIG. 1(e) with the addition of an opaque layer betweenthe electrodes that serves as a dark matrix. It is important that theopaque material be non-conductive so that adjacent electrodes remainelectrically independent.

EXAMPLE A1

A polymethylpentene substrate, available under the trade designationZeonex sold by Nippon Zeon Co., Ltd., Tokyo, Japan, was coated with anITO layer having a thickness of 80 nm (0.08 μm). The surface of the ITOlayer was then spin coated at 1500 revolutions per minute (rpm) for 60seconds with a photoresist sold by Shipley Co., Marlborough, Mass. underthe trade designation Shipley Resist 827. The coating was then dried at105° C. for 30 minutes. The photoresist was then exposed to ultravioletradiation at a wavelength of 365 nm and intensity of 9.8 mW/cm² for 30seconds through a mask having 75 μm wide parallel line apertures. Thephotoresist was then immersed for 70 seconds in a developing agent soldby Shipley Co. under the trade designation Shipley Developer 354. Thesample was then rinsed and dried. At this point, the sample wasanalogous to that shown in FIG. 1(b). The surface was then shadow coatedusing a collimated beam of Cr metal at an angle of incidence of 7°. TheCr coating was 200 nm (0.2 μm) thick. The sample was then immersed in a10% HCl solution for 70 seconds to remove the exposed ITO layer. Theexcess resist was removed by subsequent immersion in an acetoneultrasonic bath. The resulting substrate element was analogous to thatshown in FIG. 1(e).

B. Second Embodiment

Another embodiment of the method of the present invention is shown inFIGS. 2(a)-(g). First, a substantially transparent display substrate 32is provided as shown in FIG. 2(a). Substrate 32 may be of any materialsuitable for use in an electronic display substrate element as discussedpreviously.

Strips 34 of a resist material are formed on the substrate as shown inFIG. 2(a). The strips are formed by conventional lithography techniques.Typically, the entire substrate is covered with the resist material bylamination or by coating the material in a liquid form. The resistmaterial is patterned by selective exposure to light at a givenwavelength and intensity, followed by removal of the activated materialas discussed above in part A. The resist strips 34 of FIG. 2(a) may becompared to the resist strips 24 of FIG. 1(b). Whereas in FIG. 1(b) thewidth of the resist strips was chosen to correspond to the desired widthof the electrodes on the finished display substrate element, converselythe width of resist strips 34 in FIG. 2(a) correspond to the desiredwidth of the gap between electrodes on the finished display substrateelement.

Next, a metal 36 is deposited over the substrate and resist strips asshown in FIG. 2(b). The metal may be deposited by any suitable techniquesuch as vapor deposition, sputter deposition, chemical plating,electroplating, or other methods known in the art. The metal ispreferably coated to substantially cover the surface of the substrateand the resist strips.

Next, a second resist material 38 is coated over the metal coating asshown in FIG. 2(c). Preferably, the second resist is coated as a liquidso that a meniscus forms on the metal coating in the comer area formedby resist strips 34. For example, a suitable liquid resist material maybe the one sold by Shipley Co.,. Marlborough, Mass. under the tradedesignation Shipley 1818. The meniscus of the liquid resist materialensures that additional resist material is built up in the comers on themetal coating. Thus, when the second resist material is etched back toexpose the metal coating on top of the resist strips and between theresist strips, excess second resist material 38 remains at the comers onthe metal coating as shown in FIG. 2(d). The etch back of the secondresist material is preferably performed using conventional reactive ionetching or plasma etching techniques.

Next, the metal coating is etched with the remaining second resistmaterial acting as an etch barrier. Etching removes the entire metalcoating except those portions covered by the remaining second resistmaterial. This leaves a continuous strip, or line, of metal coatingdisposed along each edge of the resist strips. The remaining secondresist material is then stripped from the substrate element to leavesubstrate 32, resist strips 34, and remaining strips of metal 36adjacent to the edges of the resist strips as shown in FIG. 2(e).

Next, a transparent conductive material 39 is deposited over thesubstrate element to substantially cover the exposed areas of thesubstrate as shown in FIG. 2(f).

Finally, resist strips 34 are removed from substrate 32, along with anyresidual transparent conductive material residing thereon, to leavesubstrate 32 having independently addressable electrodes made up ofparallel strips of transparent conductive material 39, each strip havingauxiliary metal strips 36 disposed along its edges as shown in FIG.2(g).

The embodiment of the method of the present invention described in thispart and shown in FIGS. 2(a)-(g) provides distinct advantages. First,because two auxiliary metal strips are formed for each electrode, theelectrodes have a higher conductivity than for one auxiliary metalstrip, thus allowing for a thinner transparent conductive layer in theelectrode, and hence improving transmission through the substrateelement. In addition, the resistivity of the electrodes is tunable bycontrolling the amount of etch back of the second resist material—moreetch back means more metal coating exposed which means more metalcoating is removed upon etching, resulting in higher resistivityauxiliary metal strips. Second, the auxiliary metal strips are formedoff of pre-patterned resist strips and not off of pre-patternedtransparent conductive electrodes. This allows the metal strips to bepatterned independently from the transparent conductive electrodematerial, therefore allowing fabrication of thinner electrodes. Third,because the final step of this embodiment of the method of the presentinvention is a removal step rather than a metal addition step, there isno risk of adding excess metal that may short out adjacent electrodes.

EXAMPLE B1

This example describes the process shown in FIGS. 2(a)-(g) on amicroreplicated substrate where the resist strips 34 are parallel ridgesthat are integral with the substrate. The use of microreplicated plasticsubstrates in liquid crystal display devices is explained in U.S. Pat.No. 5,268,782 to Wenz. Briefly, a liquid crystal display substrate maybe imparted with a series of microreplicated parallel ridges, each ridgerising to the same height in a range of approximately 1 μm to 20 μm fromthe surface of the substrate depending on, for example, the type ofliquid crystal used. As explained in co-pending U.S. patent applicationU.S. Ser. No. 08/999,287, U.S. Pat. No. 6,077,560 these ridges may beused to further pattern the substrate without the used of further maskalignment steps.

A sample of a microreplicated plastic liquid crystal display substratewas coated with an approximately 2000 Å(0.2 μm) thick copper film viasputter deposition. Next, a thin photoresist layer was coated over thecopper using the 180R knurl roll on a Yasui-Seiki microgravure coaterCAG-150. The photoresist solution was prepared by diluting Shipley 1818resist in methyl ethyl ketone to obtain a 20% solids solution. Thephotoresist coating was dried at 100° C. in an air impingement oven.After drying, a photoresist coating approximately 1 to 1.5 μm thick wasobtained. However, the coating thickness along the edges of thesubstrate ridges was much higher due to surface tension effects duringapplication. The photoresist coating was then etched in an RF oxygenplasma to remove the resist coating from the center of the channels. Theetching time was adjusted such that photoresist remained along the edgesof the ridges after etching. The oxygen plasma was generated at 100mTorr pressure, 100 sccm flow rate, and 120 Watts of RF power. Theetching time under such conditions was 3 minutes. Following the etchingof the photoresist, the exposed copper was etched with a 10% H₂SO₄solution at about 50° C. for 2 minutes in a spray system. Next, theremaining resist was stripped using a 4% NaOH solution at about 45° C.also by spraying. The resulting substrate had copper lines along theridge edges in a manner analogous to FIG. 2(e). Next, the sample wascoated with ITO followed by a coating of Shipley 1818 photoresist usingthe Yasui-Seiki coater to obtain a resist thickness of approximately 2.5to 3.0 μm in the channels. The sample was then etched in an oxygenplasma under the conditions described above for 2 minutes. This resultedin the removal of the resist from only the tops of the ridges, leavingresist material in the channels between ridges to protect the ITO. Theexposed ITO on the tops of the ridges was then etched with a 10% H₂SO₄solution at about 45° C. for 1 minute. Finally, the remaining resist wasstripped using a 4% NaOH solution at about 50° C. The sample producedwas much like that shown in FIG. 2(g), except that the microreplicatedparallel ridges existed where gaps are shown in FIG. 2(g) between theITO strips.

EXAMPLE B2

This example describes the fabrication of copper bus lines on flatsubstrates. The process steps for the fabrication of bus lines on flatsubstrates are very similar to those in Example B1 for microreplicatedribbed substrates. First, a negative photoresist (Hercules SF 206) waslaminated to a 100 μm PET film and patterned into lines using standardlithography processes. The result was as shown in FIG. 2(a). Afterpatterning the photoresist, the substrate was coated with copper as inFIG. 2(b). Next,.Shipley 1818 photoresist was coated on the substrateusing the Yasui-Seikin microgravure coater to obtain a sample as shownin FIG. 2(c). The Shipley resist was then plasma etched to remove theresist from the substrate except at the edge of the strips of theHercules resist, as shown in FIG. 2(d). The exposed copper layer wasthen etched in a 10% H₂SO₄ solution at about 50° C. for 2 minutes. TheShipley resist was subsequently removed in an oxygen plasma to obtain asample as shown in FIG. 2(e). The substrate containing copper lines wasthen coated with ITO as in FIG. 2(f). Stripping of the Hercules resistin a 4% NaOH solution at about 45° C. for 2 minutes resulted inpatterned ITO lines on the substrate, as in FIG. 2(g).

C. Third Embodiment

Still another embodiment of the present invention is shown in FIGS.3(a)-(f). First a substrate element is provided comprising asubstantially transparent substrate 40, a TCO layer 42, and a contiguousmetal layer 44 having a periodic array of holes. Although the metallayer in FIGS. 3(a)-(f) is positioned on top of the TCO layer, themetallic layer could also be disposed between the substrate and the TCOlayer. For each step in the process shown in FIGS. 3(a)-(f), a top viewand a cross-sectional view are shown. The cross-sectional views aretaken along their respective lines 3 b, 3 d, and 3 f.

As seen in the top view of FIGS. 3(a) and (b), the metal layer 44 ischaracterized by a periodic array of holes 50. The metal layer is formedby depositing a uniform metal layer by known techniques such as vapordeposition or sputtering. A resist material is then coated on the metallayer and exposed to light through a mask having the desired pattern.The activated resist is then washed away so that the remainingphotoresist coating forms an array of holes that expose the underlyingmetal layer. The exposed metal is then etched and the photoresist isremoved. The resulting metal layer is a contiguous layer having aperiodic array of holes. Because the independently addressabletransparent conductive electrodes have not yet been formed at thisstage, there is no need to align the photoresist mask in any particularorientation.

The shape of the individual holes and their arrangement in a periodicarray may take many forms, as discussed below. The shape of the holesand the arrangement of the array will affect both the conductivity ofthe metal layer and the transparency of the substrate element. Forsimplicity, a metal layer having a hexagonal array of circular holes isshown in FIGS. 3(a)-(f).

Next, strips 46 of resist material are provided as shown in FIGS. 3(c)and (d). The width of the resist strips 46 corresponds to the width of apixel in a finished display. The resist strips 46 are provided bycoating the surface with a resist material and drying it to form acoating. The resist coating may then be exposed to light of a certainwavelength to activate the photo-initiators in the resist. The resistcoating is exposed to the light through a mask so that only certainareas are activated. Depending on whether the resist is a positiveresist or a negative resist, the activated or non-activated areas may beremoved by rinsing in a solution. The resulting resist coating is aplurality of resist strips separated by a distance corresponding to theinactive area of the display between pixels. This result is shown inFIGS. 3(c) and (d). The resist material may be any resist materialdesired that is compatible with the electrode material. There are manycommercially available formulations of resist materials, and theprocedure for their coating, drying, activation, and removal will beknown to the user when a particular resist material is chosen.

Next, the areas of the metal and TCO layers not covered by the resiststrips are removed by etching. The resist coating may then be removed toproduce the substrate element shown in FIGS. 3(e) and (f). As may therebe seen, the substrate element comprises a substantially transparentsubstrate 40, and independently addressable electrodes each having a TCOlayer 42 and a contiguous metal layer 44 characterized by a periodicarray of holes 50.

The contiguous metal layers of the independently addressable electrodesserve multiple functions. First, the metal layers increase theconductivity of the electrodes relative to bare TCO electrodes. Second,the periodic array of holes allows electrical continuity in the metallayer while minimally affecting the transparency of the substrateelement. Third, because a metal layer covers the entire width of anelectrodes, there is no need for a precision-alignment step. Rather, themetal and TCO layers are removed together using the same etch mask sothat a separate mask for the metal layer that would require a precisionalignment step need not be used. Finally, the structure of the metallayers provides mechanical stability. In many applications, largedisplays having polymeric substrates may be desired. These substratesare often flexible, leading to inadvertent bending, twisting, ordistorting during processing. Such stress may cause TCO electrodes orsingle auxiliary metal strips to crack. However, the metal layer havinga periodic array of holes provides multiple conductive paths, thusdecreasing the possibility of failure of an electrode due to crackingand loss of electrical conductivity.

Examples of different hole shapes and array configurations that may beused in the metal layers are shown in FIGS. 4(a)-(e). The percentage oflight transmitted through these configurations is determined by theareal density of holes.

An array of diamond shaped holes is shown in FIG. 4(a). The pattern isdefined by holes have four sides of equal distance a, a perpendiculardistance d separating the holes, and the angle θ. The transmissionpercentage T of this configuration is:$T = {\frac{a^{2}\sin^{2}\theta}{\left( {{a\quad \sin \quad \theta} + d} \right)^{2}}.}$

Thus, T approaches 100% as d gets smaller.

A square array of square holes is shown in FIG. 4(b). Each square holehas sides of length a, and the distance between the holes is b. Thus,the transmission of this pattern is given by:$T = {\frac{a^{2}}{\left( {a + b} \right)^{2}}.}$

Again, as b is made smaller, the transmission of this pattern approaches100%.

In FIG. 4(c) a hexagonal array of circular holes is shown. The radius ofeach circular hole is r and the center-to-center distance betweencircular holes is d. This pattern has a transmission of:$T = {\frac{\pi \quad r^{2}}{\left( {\sqrt{3}/2} \right)d^{2}}.}$

The maximum transmission of this pattern is 90.7%, namely when d=2r.

A regular array of equilateral triangular hole pairs is shown in FIG.4(d). The distance a is the distance between adjacent triangle apexes,the distance b is the distance between adjacent triangle bases, and d isthe length of a side of a triangular hole. The transmission of thispattern is:$T = {\frac{\sqrt{3}d^{2}}{\left( {d + {2a}} \right)\left( {{\sqrt{3}d} + {2b}} \right)}.}$

Thus, reduction of either or both of a or b results in highertransmission.

Finally, a hexagonal array of hexagonal holes is shown in FIG. 4(e). Thelength of each side of a hexagonal hole is a, and the distance betweenholes is b. The transmission of this patter is given by:$T = {\frac{3a^{2}}{\left( {{\sqrt{3}a} + b} \right)^{2}}.}$

When b is decreased, T approaches 100%.

EXAMPLE C1

A diamond grid mask was used as shown in FIG. 4(a) with a=51 μm, d=10μm, and θ=45°. The transmission, or aperture ratio, of this mask was61%. To make a patterned metal coating, first copper was evaporated ontoa PET substrate to a thickness of 100 nm (0.1 μm). The sheet resistanceof the copper coating was measured to be 0.74 Ω/square. The coppersurface was then spin coated with Shipley photoresist 818 at 2000 rpmfor 40 seconds and dried at 105° C. for 30 minutes. The photoresist wasthen exposed through the diamond grid mask to ultraviolet light at awavelength of 365 nm and an intensity of 12.9 mW/cm² for 15 seconds. Theactivated areas of the photoresist were rinsed away to exposediamond-shaped portions of the copper surface. The exposed copper wasetched with a diluted solution of FeCl₃ to make the diamond-shapedholes. Due to over-etching, the width of the copper pattern, d, wasapproximately 5 μm in this example rather than the ideal 10 μm. Thephotoresist was then removed and the sample was coated with ITO. Theresultant sheet resistance was measured to be 25 Ω/square, and theoptical transmission was measured to be 70% in the visible spectrum,including two surface reflection losses, whereas the aperture ratio ofthe pattern was 78%.

EXAMPLE C2

The same procedure was followed as in Example C1, except that the coppercoating thickness was 200 nm (0.2 μm) and the over-etching wasminimized. The resulting sheet resistance was 4.3 Ω/square, and theoptical transmission was 65%.

EXAMPLE C3

The same procedure was followed as in Example C1, except that the maskused had the following dimensions: a=190 μm, d=10 μm, and θ=45°. Also,the metal used was aluminum and was coated to a thickness of 100 nm (0.1μm). The resulting sheet resistance after patterning and ITO coating was10 Ω/square with an optical transmission of 78%.

What is claimed is:
 1. An electronic display device comprising asubstrate element that is substantially transparent to visible light,the substrate element comprising a transparent base layer, a pluralityof independently addressable transparent electrodes disposed on the baselayer, and a contiguous metallic coating associated with eachtransparent conductive electrode to increase the conductivity of theassociated transparent conductive electrode, wherein the contiguousmetallic coating comprises a periodic array of holes arranged to allow asignificant amount of visible light to be transmitted through thesubstrate element.
 2. The substrate element of claim 1, wherein thetransparent conductive electrodes comprise a transparent conductiveoxide.
 3. The substrate element of claim 2, wherein the transparentconductive oxide is indium tin oxide.
 4. The substrate element of claim1, wherein the metallic coating comprises Cr, Cu, Ag, Au, Ni, W, Al, Pt,Ti, Fe, Sn, or combinations and alloys thereof.
 5. The substrate elementof claim 1, wherein the periodic array of holes is selected from thegroup consisting of: (a) a two-dimensional array of diamond-shapedholes, (b) a hexagonal array of circular holes, (c) a square array ofsquare holes, (d) a two-dimensional array of triangular hole pairs, and(e) a hexagonal array of hexagonal holes, and combinations thereof.